Together with introduction of a natural energy power generation apparatus which utilizes natural energy such as wind-power generation and solar power generation into a power line, there is the possibility that the power system associated with the apparatus is adversely affected by frequency variation or voltage variation. As one of countermeasures, a method has been proposed in which a power storage apparatus is provided to a natural energy power generation apparatus to suppress the voltage variation of a power line of the natural energy power generation apparatus. For example, a power storage apparatus for compensating for output variation of a natural energy power generation apparatus is disclosed in Patent Document 1. In the power storage apparatus, an assembled battery system which includes a plurality of series battery units of lithium batteries connected in parallel to each other and a control apparatus is configured. The assembled battery system changes over between charge and discharge in response to output variation of the natural energy generation apparatus. Further, each of the series battery units includes a current control element which is controlled in response to a voltage, a temperature or an internal resistance of the battery unit, or these of a battery cell which constitute the battery unit in order to prevent a dispersion of a voltage among each unit.